This invention relates generally to single phase permanent magnet motors and more particularly to a coil assembly for sensorless rotor angular position control of a single phase permanent magnet motor.
Conventional hermetic refrigerator compressors, for example, typically use fixed speed single phase induction motors. On the other hand, variable speed operation of motors is advantageous for improving efficiency. Although conventional three phase permanent magnet (PM) motors, such as those being used for heating, ventilating, and air conditioning applications, are capable of variable speed operation, they are more expensive than single phase PM motors which require fewer power semiconductor switches and associated gate drivers.
Single phase PM motors require a suitable current commutation signal synchronized with the rotor position for proper operation. In single phase applications, a Hall-effect position sensor is typically used to detect the rotor position and thereby control the motor. Such single phase motors having a Hall-effect sensor, however, have several disadvantages. For example, the reliability of Hall sensors is uncertain in environments such as refrigerator compressors. Further, position sensors require additional leads which are not desirable in certain environments.
In order to avoid the use of a Hall sensor or other rotor position sensor, various sensorless control schemes have been developed for PM motors. In three phase PM motors under normal operation, there are times when one phase is open-circuited and has no current flowing in it. Under such conditions, the terminal voltage is equal to the back EMF voltage and can thus be sensed directly. Single phase motors, however, do not have natural intervals where the phase current remains zero for any length of time, and this approach is therefore not applicable.
For three phase motors, even if the phase current is non-zero, the back EMF voltage can be calculated by modeling the motor as a resistance, inductance, and back EMF voltage source, as described by M. Jufer, "Back-EMF Indirect Detection for Self-Commutation of Synchronous Motors," European Power Electronics Conference, 1987, pp. 1125-29. Although this technique can be applied to single phase PM motors, it is difficult to provide a controllable preferred direction of rotation. Thus, the motor can start in either direction, depending on the initial rotor angular position. Fan and compressor drives generally are designed to operate in only one direction of rotation, so control over the rotation direction is critical. Furthermore, the required knowledge of the motor parameters is not always available and is subject to production and operating variations.
Commonly assigned application Ser. No. 08/680,010, filed Jul. 15, 1996, the entire disclosure of which is incorporated herein by reference, describes a quadrature winding suitable for generating a position signal in a single phase motor. Further improvements, however, are desired.